Digital pulse width modulators (PWM) are in widespread use in entertainment electronics and other areas. Existing digital pulse width modulators require a high time resolution of the pulse widths, which, for example in the audio range of 0 to 20 kHz, necessitates a clock frequency of approximately 100 MHz. According to Erick Bresch, Wayne T. Padgett, “TMS320C67-Based Design of a Digital Audio Power Amplifier Introducing Novel Feedback Strategy”, relatively strong non-linear distortions occur in the case of high modulation in a digital PWM.
When sigma-delta modulation (SDM) is used, only a low clock frequency, of for example 2 to 4 MHz, is required for an audio signal, but the output signal then tends to be a pulse-density-modulated signal, which is unsuitable for example for a Class-D amplification on account of the signal-dependent pulse density, since, in the case of pulses that are not ideal, this leads to non-linear distortions. In particular, according to A. J. McGrath, M. B. Sandler, “Power digital to analogue conversion . . . , Electronic Letters, issue 31, No. 4, 1995, a constant pulse frequency is not ensured in the case of sigma-delta modulation.
Class-D amplifiers have in comparison with A, AB amplifiers a much lower power loss and are typically driven by PWM signals. It is known that digital pulse width modulators require a high time resolution of the PWM signal in order to minimize distortions caused by the time quantization. To date, a digital input signal is reduced with the aid of a multi-bit sigma-delta modulator in the amplitude resolution with, for example, 8 bits for a dynamic range greater than 80 dB and then the quantized signal with low resolution is fed to a pulse width modulator. On the one hand, as already mentioned, this requires a high clock frequency of more than 100 MHz on account of the relatively high time resolution of the pulse width signals (8 bits correspond to 256 different pulse widths), and on the other hand the pulse-width-modulated signal generated in this way is not free from non-linear distortions, since it is not the PWM signal but the amplitude-quantized signal that is fed back in the control loop, the two signals in the baseband, i.e. in the audio range of for example 0 to 20 kHz, not being completely identical. Therefore, the quantization noise is not optimally suppressed for the PWM signal by the control loop in the sigma-delta modulator.
Apart from great complexity of its circuitry, according to Jorge Varona, ECE University of Toronto, “Power Digital to Analog Conversion Using Sigma Delta and Pulse Width Modulations”, a known method for digital PWM likewise requires a high operating clock frequency. In FIG. 6, a typical configuration for a digital pulse width modulator is represented. For the linearization of the PWM signal 15′, the digital input signal 1 is extremely highly interpolated in an interpolation filter 10 and then limited in the amplitude resolution by means of a noise shaper 23 in the sigma-delta modulator. Since, however, the noise shaper 23 does not process the quantized PWM signal 15′ but only the quantized amplitude signal before the pulse width modulation in a pulse width modulator 24, the actual quantization noise and the non-linearities of the time-quantized PWM signal 15′ can only be suppressed sub-optimally. The digital PWM signal 15′ is subsequently typically filtered in a post-filter 16, preferably after the amplification of the signal in an amplifier device (not represented).